This invention relates generally to magnetic resonance imaging (MRI) and, more particularly, the invention relates to the correction of MRI distortion due to magnetic field inhomogeneity.
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
In obtaining magnetic resonance signals, a constant magnetic gradient can be employed as in 2-D Fourier Transform (2DFT) or Spin Warp imagery, or time varying magnetic gradients can be employed as in spiral k-space scanning. Inhomogeneity and chemical shift are significant challenges for any pulse sequence for MRI data acquisition. In 2DFT off resonance causes a local shift in the direction of the readout gradient, while in non-2DFT acquisitions (time-varying gradients), off-resonance causes local blurring. There are two important sources of inhomogeneity. The largest effect is due to the 3.4 ppm chemical shift between fat and water, which can be avoided by exciting only water, for example with a spectral-spatial pulse. The second effect is due to main-field inhomogeneity, which is typically on the order of 1 ppm. This effect is especially important for multi-slice sequences where off-center slices are difficult to shim. To minimize inhomogeneity artifacts Noll et al. proposed frequency and time segmented reconstruction algorithms, see U.S. Pat. No. 5,311,132. These algorithms include the acquisition of a field map and multiple reconstruction steps by a gridding procedure. Although this technique works well, it possesses two major drawbacks in that it is time consuming because of the multiple reconstruction steps and it fails in places where the field changes abruptly.
The present invention is directed to a fast and robust correction method that corrects only for the linear terms of an acquired field map.